This invention relates generally to optical measurement probes and, in particular, to a method and apparatus for focusing such a probe.
Induced radiative effects such as Raman scattering and fluorescence have become extremely valuable tools associated with the non-destructive determination of molecular constituents. Optical probes for such purposes are being employed in on-line process control in increasing numbers. These probes are often installed directly into the process stream or reactor, thus posing a potential safety hazard.
Free-space optical spectroscopy probes used in immersed applications typically involve imaging an optical sampling beam through a window bonded in the wall of a containment vessel. The window can be any material transparent to wavelengths of interest, though the most popular material being sapphire.
Many state-of-the-art spectroscopic fiber-optic probes used in laboratory and process analysis applications are based on confocal imaging. This is particularly true of laser-induced spectroscopy (including Raman), wherein the relayed optical images of both the fiber 10 carrying the excitation light and that of the collection fiber 14 are combined into a single probe beam by a combiner 20, as shown in the prior art arrangement of FIG. 1. The excitation and collection beams are respectively collimated and focused by elements 12 and 16, and the combined probe beam is focused by a lens 106 into/onto a sample 104, typically through window 106 which, again, is usually sapphire.
The position of the relayed, combined image of the fibers relative to the window is critical for some applications. In general, it is desirable to construct a probe whose fixed focal position is best suited to the largest cross-section of a particular application. There are two generally classes of samples: those which are transparent and those which are opaque. For transparent samples, it is best to focus at a point which is well into the material, shown in FIG. 2 as point 200. For opaque samples, however, it is advantageous to focus directly on the surface (at the window to sample interface), as shown in FIG. 3.
Broadly, this invention resides in a method of positioning a focused image within a sampled medium in a optical measurement probe of the type wherein a focused sampling image is transmitted through a window having a surface facing a sampled medium. According to the method, a test medium is measured, preferably through the window. The magnitude of an optical signal associated with the test medium is then compared to the magnitude of an optical signal associated with the window, and the result of the comparison is used to position the focused image. Typically, the magnitudes of optical signals are representative or Raman scattering or another wavelength-selective radiative sampling process such as fluorescence detection.
The method is not limited in terms of window composition, and is compatible with sapphire windows commonly used in industry. Nor is the invention limited in terms of test medium though, in the preferred embodiment a fluid hydrocarbon such as isopropyl alcohol is used. The signal comparison is preferably a ratio which may be used to position the focused image substantially proximate to the surface of the window for a sampled medium which is substantially opaque or away from the surface of the window for a sampled medium which is substantially transparent. As a further alternative, the magnitude of optical signal of the test medium and the magnitude of optical signal of the window may be roughly the same as a compromise to accommodate both opaque and transparent sampled media.
In many systems, a focusing lens is supported near the window, such that the step of using the result of the comparison to position the focused image includes moving the lens relative to the window to adjust the position of the focused image.